spl_mmc/CONFIG_SPL_OS_BOOT: Allow environment to determine what to boot

We add two new environment variables, falcon_args_file and
falcon_image_file, which when set will override the compiled in default
values for falcon mode.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/doc/README.falcon b/doc/README.falcon
index bccf6c9..82a254b 100644
--- a/doc/README.falcon
+++ b/doc/README.falcon
@@ -89,6 +89,10 @@
 boot_os : 		Set to yes/Yes/true/True/1 to enable booting to OS,
 			any other value to fall back to U-Boot (including
 			unset)
+falcon_args_file :	Filename to load as the 'args' portion of falcon mode
+			rather than the hard-coded value.
+falcon_image_file :	Filename to load as the OS image portion of falcon
+			mode rather than the hard-coded value.
 
 Using spl command
 -----------------